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Thanks for your reply.
I measured Pin 30 on uC (the 8051 replacement is an Atmel AT89C55WD) at 1us but signal still showed aliasing. Did that on the OK DME too, FWIW.
I'm surprised to see in the reset signal what looks like the inverted saw tooth of Pin 6/S890 lm139. Perhaps it is not aliasing at all?
I removed the ADC which did not change the reset signal
With it removed I replaced the EPROM, S702 transparent state latch,
S704 hex buffer/converter
and S705 - 74LS02 gate and checked the Pin 9 reset signal on the uC 8051 in between each and every replacement.
Unfortunately it never changed from the signal shown in the last picture of it, I.e. square wave going high at ~30 Hz.
Putting the ADC back didn't change it either.
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